In computing systems, such as desktop computers, portable computers, personal digital assistants (PDAs), servers, and others, storage devices are used to store data and program instructions. One type of storage device is a disk-based device, such as a magnetic disk drive (e.g., a floppy disk drive or hard disk drive) and an optical disk drive (e.g., a CD or DVD drive). Disk-based storage devices have a rotating storage medium with a relatively large storage capacity. However, disk-based storage devices offer relatively slow read-write speeds when compared to operating speeds of other components of a computing system, such as microprocessors and other semiconductor devices.
Another type of storage device is a solid state memory device, such as a dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, and electrically erasable and programmable read-only memory (EEPROM). Although solid state memory devices offer relatively high read-write speeds, usually on the order of nanoseconds, they have relatively limited storage capacities.
With improvements in nanotechnology (technology involving microscopic moving parts), other types of storage devices are being developed. One such storage device (referred to as a “probe-based storage device”) is based on atomic force microscopy (AFM), in which one or more microscopic scanning probes are used to read and write to a storage medium. Typically, a scanning probe has a tip that is contacted to a surface of the storage medium. Storage of data in the storage medium is based on perturbations created by the tip of the probe in the surface of the storage medium. In one implementation, a perturbation is a dent in the storage medium surface, with a dent representing a logical “1,” and the lack of a dent representing a logical “0.”
Dents are typically formed by heating a probe tip to a write temperature to melt regions on a storage medium, with the melted regions forming the dents. During read operations, the probes are also heated to a read temperature (usually lower than the write temperature). Detection of whether a probe has encountered a dent is based on the detected resistance of the probe, which resistance is proportional to the temperature of the probe. If the probe tip is engaged in a dent, the probe is brought into closer proximity to the surface of the storage medium. The closer proximity of the probe to the storage medium enables greater dissipation of heat from the probe to the storage medium, which causes the temperature of the probe to drop. The drop in temperature causes the resistance of the probe to change. Circuitry in the storage device is able to detect the change in resistance to thereby determine if dents are present in respective storage cells.
Often, the density of dents formed in a storage medium can vary by region of the storage medium. Thus, in some regions of the storage medium, there may be relatively high densities of dents. In other regions of the storage medium, there may be relatively low densities of dents. Heat dissipation from a probe is usually lower when the probe is in a region with a relatively high density of dents. Thus, as the probe moves about relative to the storage medium, the heat dissipation and therefore the probe temperature may vary.
The variations in densities of dents across a storage medium, and the consequent variations in probe temperatures during read operations, contribute to undesirable additive noise to readback signals. A readback signal is a signal produced by the probe during a read operation. The noise added to readback signals can lead to an increase in read data errors.